I suggest you get the one that's tiny rather than massive. Once you get the hang of it, then you can work your way up to larger ones. Have fun. It's great! Enjoy it and consider yourself lucky to find a lady willing to do you with a strap on.

There is a product called Feeldoe that has a bulb that goes into the woman's vagina and is held in place firmly while allowing her to use the cock end for penetrating a partner. You can find them online or in stores.